1. Field of the Invention
The present invention relates to a method for controlling the uniformity of particle size in powder manufacturing processes. More particularly, the invention relates to a method for controlling uniformity of particle size by controlled Ostwald ripening at large levels of supersaturation.
2. Background of the Invention
Many production and industrial processes require the use of powders, either in dry form or in slurries. For example, the DuPont Chemical Company estimates that 80% of its products involve the use of either particulates, or particulate intermediaries. It is important that these powders react as well and as predictably as possible for insuring the success of processes in which they are used. Much effort has been devoted to the important problem of synthesizing particulates having uniform sizes. Powders having a small particle size variance tend to react uniformly. On the other hand, powders having a larger size variance amongst their crystals, have the tendency for smaller crystals in the system to react faster than larger crystals. The difference in reaction rates results in inferior reactions and inferior products. Additionally, uniform powders have better flow properties. These powders are less likely to plug filters with crystals from the fine part of the size distribution, or to lose product from the coarse part of the distribution by settling. The firing properties of such uniform powders are more uniform, leading to greater strength for ceramics. Optical coatings using more uniform crystals are more transparent. Abrasives with more uniform powders used in polishing compounds are superior, leading to smoother polished surfaces. In the magnetic recording industry, a uniform grain size distribution of nanocrystals used in making magnetic tapes and disks would dramatically enhance the signal to noise ratio. Many other products, including dyes, polymers and photographic films, would benefit from improved control of particle size during synthesis.
Two mechanical methods of powder manufacture are commonly used within the powder manufacturing art to control crystal size. These methods are grinding and sieving. Both of these methods can be used to control the mean size of a batch of crystals. However, these methods lead to a broad distribution of sizes amongst the individual crystals.
The prior art includes other attempts to improve the control of both particle size distribution and mean particle size within a particulate batch. Prior art patents of interest include U.S. Pat. No. 5,716,419 (Larsen), U.S. Pat. No. 5,254,453 (Chang), U.S. Pat. No. 4,067,739 (Lewis), and U.S. Pat. No. 3,900,292 (Fairchild). Briefly considering these patents, the Lewis patent discloses a method for the production of a silver halide emulsion containing crystals with a narrow size distribution. The method involves Ostwald ripening followed by a crystal growth stage and produces a monosize silver halide emulsion in which the majority of the crystals are of the twin octahedral type. The Fairchild patent discloses a process for controlling the size distribution of crystals by maintaining a fixed ratio between the rate in which the crystals are added to, and products slurry withdrawn from, a crystallizer. The Larsen patent discloses a method of producing large crystals with a narrower and more uniform size distribution involving the use of lecithin or lecithin-containing compositions in aqueous solutions containing mineral salts. The Chang patent discloses a process for the preparation of tabular silver halide grains with a narrow size distribution and a lower percentage of fine grains involving digesting nucleated grains in a basic silver halide solvent with a specific concentration. The basic solvent is neutralized when the digestion is complete.
In accordance with the invention, a method is provided for the manufacture of crystals having a uniform mean size, the method comprising the steps of: a) forming a solution which is supersaturated with respect to the crystals and Ostwald ripening the crystals in the solution at a level of supersaturation that produces an Ostwald universal steady state crystal size distribution shape wherein the crystals have approximately the same size; b) after Ostwald ripening ceases, further growing the crystals by supply controlled growth; and c) terminating the supply controlled growth at a desired mean crystal size.
Advantageously, the supply controlled growth is terminated by a reaction poisoning step, although other termination methods can be used.
In one preferred embodiment, the crystals are calcite crystals, although the invention is applicable to a great many different minerals. Advantageously, in the preferred embodiment, the level of supersaturation is such that the solution ion activity product divided by the calcite solubility product (omega) is greater than 100.
Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.